Why does a bigger plenum allow the same results with lower reg pressure?

[Guest]

During the brief time the valve is open, a pressure drop occurs in the plenum. With a larger volume of air at the ready, there is less of a pressure drop. Therefore the average pressure available to accelerate the projectile is higher.

Short and to the point. This pretty much sums up that thread. Thanks for the explanation

Yes, my vote for nervoustrig, the other answers given are wrongish or incomplete.

Well you should help out those of us who were wrongish or incomplete by making the appropriate corrections or additions. It is a very hard topic to cover completely in a single post. I am sure your input will be very illuminating.

 

[Guest]

How can an old unregulated 1500psi max fill Mac1 USFT do what it does?

I think of the long tubed (unregulated) USFT sort of as the original power plenum, that whole tube is the plenum, ie the pressurized air directly available for each shot. So, the gun can get a nice consistent string in the 19.5fpe area by relying on lots of low pressure air, vs a smaller amount of high pressure air. 

At some point in my digging I kinda remember coming across someone estimating the tube volume of the full-length version like mine to be roughly 800ccs. That's a lot of air at the same pressure. So, no regulator, per se, but lots of immediately available air for the shot, and all that air is in one "compartment" of the same pressure (vs a regulated gun that has two "compartments" of different pressure air: a certain, and relatively, small volume of the "correct" pressure of air between the reg and the back of the pellet, and another larger compartment of non-regulated air upstream of the regulator.

Nailed it. Not so much to do with the small port and so much more to do with the volume of the "plenum". A .22 barrel that is 500mm long has a total volume of 1.4 cubic cm. So an 800 cc "plenum" contains enough air at 1 BAR to fill that barrel 571 times. At 100 BAR it can fill that barrel 57100 times...

Bob Sterne is a treasure. I would not understand any of this if it were not for him taking the time to get me started back when I frequented GTA. There may still be a thread over there where he helped me work through tuning a BSA Lonestar. I'm happy to see he has found other places to grace with his knowledge these days.

 

[nervoustrig]

No doubt the large reservoir is a big contributor but so too is small porting when tuning for long strings at modest power levels with a conventional (unregulated) PCP.

Very much the opposite thing one wants when setting up a regulated gun.

 

[xyon]

Well you should help out those of us who were wrongish or incomplete by making the appropriate corrections or additions. It is a very hard topic to cover completely in a single post. I am sure your input will be very illuminating.

Sorry. In retrospect, I think I should have just said which post I liked best and not said anything negative about the other posts.

 

[Odoyle]

Thanks all for the insight (low pressure unregulated USFT stuff)

 

[Scotchmo]

Pressure x volume = energy

Increase the plenum pressure, or increase the plenum volume and you get an increase in available plenum energy. That can yield higher energy (fpe) output.

 

[Pale_Rider]

Pressure x volume = energy

Ding Ding Ding! That’s the exact answer I was looking for. Amazing how only 3 words could describe that process. Thanks for the equation

 

[markT]

So where does flow fit into this?

Seems like having a large volume of air to work with is pointless if you can't get it to where you need it quickly with laminar flow..

 

[cilami]

Will a bigger plenum allow the same results with a lower regulator pressure in a shorter barrel, like 350 mm? I would like to know if it would be worth to add a plenum to my Taipan Mutant short.

 

[nervoustrig]

In an unregulated system, valve lift and dwell are changing with the declining pressure to deliver a stable velocity. To achieve the longest useful string, it helps to use the minimum port size that provides the desired velocity _and_ allows charging to the full fill pressure (i.e. don’t have to underfill to catch the beginning of the useful portion of the bell curve). That will give you the longest useable pressure range, which equates to being able to extract a larger amount of the energy stored in the reservoir.

 

[Guest]

Will a bigger plenum allow the same results with a lower regulator pressure in a shorter barrel, like 350 mm? I would like to know if it would be worth to add a plenum to my Taipan Mutant short.

Only to a point. I have a .25 Condor SS with a 12" barrel. I have an Altaros regulator on it. The regulator sits between the bottle and the action. The plenum on that regulator is a bit small. I can push a JSB 25.39 grain pellet at about 820 fps with the reg set at 150 bar. That will give me maybe 20 or 25 shots before I drop off reg. If I dial the power back and shoot those pellets at 780 fps I get 58 shots on reg and half a dozen more good ones after that. It takes TIME for a pellet to accelerate and short barrels just don't give enough time. PCPs really need that extra length if you are after more power. You would be better off to add a 150mm more barrel. That is pretty much universally true as far as I have been able to observe. Those short barrels really need to eat up a lot of air in order to give you higher velocities.

 

[cilami]

Oldspook, thanks for your useful response.

Ok, I can see that a larger plenum, combined with a short barrel, is not the answer for more velocity. Then, this would be my second question: with the same short barrel, if I add a larger plenum and keep the fps the same, would the shot count increase? If so, any estimate of how many more shots could be expected, relative to the original tune/configuration?

 

[Spyker5.56]

Thanks for the explanation. That’s what I was thinking. Since adding the larger plenum on my Priest 2 premium I can now set it at 130bar and get the same FPS as when before it was at 180bar. 

 

[Guest]

Oldspook, thanks for your useful response.

Ok, I can see that a larger plenum, combined with a short barrel, is not the answer for more velocity. Then, this would be my second question: with the same short barrel, if I add a larger plenum and keep the fps the same, would the shot count increase? If so, any estimate of how many more shots could be expected, relative to the original tune/configuration?

That actually depends upon whether or not you are evacuating the plenum with each shot already. If you are then shot count might increase a little. If you are not then it probably would not increase. Usually (not always) any increase in shot count will be accompanied by a drop in velocity and vice versa BUT when you start working with valve timing (hammer weight, spring preload, etc) you can optimize and see increases in both shot count and velocity.

So the Condor I referenced also had a stock hammer in it. I wanted more power in the shorter barrel than I was getting and figured I could keep the valve open a bit longer with a heavier hammer. I replaced it with a 95 gram hammer that was on the gun when I bought it. I also added an o-ring on the top hat which basically mitigates the weight of the hammer some and reduces the height of the valve opening. Prior to those two changes the gun was shooting around the same velocity level with more preload and I was dropping off reg at about 40 shots with the reg set to 138 bar. After the changes I was getting sixty shots at the same power level with practically no preload.

Short answer? Probably not unless you do some tuning on your preload and your valve. As to how many you might expect, someone else who knows what he is talking about might be useful.

Yo!

 

[cilami]

Thanks!

 

[Centercut]

Oldspook, thanks for your useful response.

Ok, I can see that a larger plenum, combined with a short barrel, is not the answer for more velocity. Then, this would be my second question: with the same short barrel, if I add a larger plenum and keep the fps the same, would the shot count increase? If so, any estimate of how many more shots could be expected, relative to the original tune/configuration?

@cilami I tend to disagree with the answer you received above. I don’t do this from theory but from experience. Here’s why I KNOW, not THINK that a plenum in a shorter barreled gun will increase power and/or efficiency (shot count)

On my .22 Cricket mini Carbine, with a 15.3 inch barrel, I was shooting the JSB 18.1 at 880 FPS and getting three magazines (42 shots) per 250b fill. I wasn’t satisfied with that and had a Cricket power plenum lying around that I had gotten for my .30 Vulcan Copperhead project. (I only ended up using one of the two that I bought). So I installed the plenum in my Cricket mini. No other changes. The gun then shot the 18.1s at 905 to 910 FPS. So I backed off the HST to shoot at 880 FPS where it is super accurate and now get 4+ magazines per 250b fill, or over 54 shots plus a few more before speed drops off. So that’s my experience with plenums and short barreled rifles...

 

[cilami]

Oldspook, thanks for your useful response.

Ok, I can see that a larger plenum, combined with a short barrel, is not the answer for more velocity. Then, this would be my second question: with the same short barrel, if I add a larger plenum and keep the fps the same, would the shot count increase? If so, any estimate of how many more shots could be expected, relative to the original tune/configuration?

@cilami I tend to disagree with the answer you received above. I don’t do this from theory but from experience. Here’s why I KNOW, not THINK that a plenum in a shorter barreled gun will increase power and/or efficiency (shot count)

On my .22 Cricket mini Carbine, with a 15.3 inch barrel, I was shooting the JSB 18.1 at 880 FPS and getting three magazines (42 shots) per 250b fill. I wasn’t satisfied with that and had a Cricket power plenum lying around that I had gotten for my .30 Vulcan Copperhead project. (I only ended up using one of the two that I bought). So I installed the plenum in my Cricket mini. No other changes. The gun then shot the 18.1s at 905 to 910 FPS. So I backed off the HST to shoot at 880 FPS where it is super accurate and now get 4+ magazines per 250b fill, or over 54 shots plus a few more before speed drops off. So that’s my experience with plenums and short barreled rifles

That is interesting! Thanks for posting your experience. Unfortunately I don't have the knowledge to understand why that "discrepancy" between the theory and your results with the Cricket. To add the larger plenum to my Taipan Mutant shortie would require to buy a unf adapter plus a new LDC plus the plenum itself...$200+. Not willing to invest that amount of money to experiment...

 

[Glem.Chally]

Oldspook, thanks for your useful response.

Ok, I can see that a larger plenum, combined with a short barrel, is not the answer for more velocity. Then, this would be my second question: with the same short barrel, if I add a larger plenum and keep the fps the same, would the shot count increase? If so, any estimate of how many more shots could be expected, relative to the original tune/configuration?

@cilami I tend to disagree with the answer you received above. I don’t do this from theory but from experience. Here’s why I KNOW, not THINK that a plenum in a shorter barreled gun will increase power and/or efficiency (shot count)

On my .22 Cricket mini Carbine, with a 15.3 inch barrel, I was shooting the JSB 18.1 at 880 FPS and getting three magazines (42 shots) per 250b fill. I wasn’t satisfied with that and had a Cricket power plenum lying around that I had gotten for my .30 Vulcan Copperhead project. (I only ended up using one of the two that I bought). So I installed the plenum in my Cricket mini. No other changes. The gun then shot the 18.1s at 905 to 910 FPS. So I backed off the HST to shoot at 880 FPS where it is super accurate and now get 4+ magazines per 250b fill, or over 54 shots plus a few more before speed drops off. So that’s my experience with plenums and short barreled rifles

That is interesting! Thanks for posting your experience. Unfortunately I don't have the knowledge to understand why that "discrepancy" between the theory and your results with the Cricket. To add the larger plenum to my Taipan Mutant shortie will require to buy a unf adapter plus a new LDC plus the plenum itself...$200+. Not willing to invest that amount of money to experiment...

Its not an experiment if its what most people would expect to happen. (More power, ability to lower reg pressure, higher shot count) a combination of the three or can tune to get 2 or 1 of those options.

 

[cilami]

Oldspook, thanks for your useful response.

Ok, I can see that a larger plenum, combined with a short barrel, is not the answer for more velocity. Then, this would be my second question: with the same short barrel, if I add a larger plenum and keep the fps the same, would the shot count increase? If so, any estimate of how many more shots could be expected, relative to the original tune/configuration?

@cilami I tend to disagree with the answer you received above. I don’t do this from theory but from experience. Here’s why I KNOW, not THINK that a plenum in a shorter barreled gun will increase power and/or efficiency (shot count)

On my .22 Cricket mini Carbine, with a 15.3 inch barrel, I was shooting the JSB 18.1 at 880 FPS and getting three magazines (42 shots) per 250b fill. I wasn’t satisfied with that and had a Cricket power plenum lying around that I had gotten for my .30 Vulcan Copperhead project. (I only ended up using one of the two that I bought). So I installed the plenum in my Cricket mini. No other changes. The gun then shot the 18.1s at 905 to 910 FPS. So I backed off the HST to shoot at 880 FPS where it is super accurate and now get 4+ magazines per 250b fill, or over 54 shots plus a few more before speed drops off. So that’s my experience with plenums and short barreled rifles

That is interesting! Thanks for posting your experience. Unfortunately I don't have the knowledge to understand why that "discrepancy" between the theory and your results with the Cricket. To add the larger plenum to my Taipan Mutant shortie will require to buy a unf adapter plus a new LDC plus the plenum itself...$200+. Not willing to invest that amount of money to experiment...

Its not an experiment if its what most people would expect to happen. (More power, ability to lower reg pressure, higher shot count) a combination of the three or can tune to get 2 or 1 of those options.

Glem.Chally,

The question would be , what result most people would expect to happen, given the 350 mm barrel? So far it is a tie 1-1 (that's assuming no valve tuning etc.. Just adding the plenum)

 

[Pale_Rider]

Will a bigger plenum allow the same results with a lower regulator pressure in a shorter barrel, like 350 mm? I would like to know if it would be worth to add a plenum to my Taipan Mutant short.

Any of you calculus guys know if there is an exact way to figure out the exact plenum size for a given barrel. I would assume you would have to find the volume of the barrel then that would be the size plenum you need?